The present invention relates to the fabrication of high strength aluminum alloy welded structures. High-strength aluminum alloys, for example, alloys with a yield strength of  greater than 60 ksi, have not been utilized for welded frame construction due to the inherently poor weldability of this class of alloys. The purpose of this invention is to provide a welded structure fabricated from high-strength aluminum.
One application for a high strength welded structure is a bicycle frame. Every year, a large number of aluminum bicycle frames are manufactured. Aluminum alloys provide a good combination of low density and good strength for this application. Bicycle frames are typically manufactured by fusion welding thin-walled tubes. The tubes are assembled as either a single frame unit or as component units that are joined together, for example by suspension members.
When selecting an aluminum alloy for use in bicycle frames, the manufacturer emphasizes the use of xe2x80x9cweldablexe2x80x9d aluminum alloys. The characteristic most used to describe weldability is resistance to hot tearing during welding. Hot tearing resistance is typically good for alloys with only small amounts of alloying additions due to their small freezing range. In addition, binary alloys that are heavily alloyed can also be resistant to hot tearing due to their ability to heal any tears with solidifying (usually eutectic) liquid. To obtain higher strengths, a relatively high level of several alloying additions results in a large freezing range, thereby increasing hot tearing during welding. Accordingly, only moderate strengths alloys are used for welded applications.
The most popular alloys used for aluminum bicycle frame manufacture include 6061 and 7005, although other 6xxx and 7xxx alloy variants are sometimes used. To a lesser extent, some 2xxx and 5xxx alloys have also been used for frame construction. 6061-T6 offers good strength, corrosion resistance and weldability. Bicycle frames constructed from 6061 tubes are typically welded in the T4 or T6 condition and subsequently re-solution heat treated, quenched and aged back to a T6 temper. Alloy 7005 is typically welded in the T6 temper and can be aged after welding without the re-solution heat treatment step. Alternatively, it can also be solution heat treated after the frame is welded. 5086-H32, has also been used for frame construction and is a non-heat treatable alloy. Of these aforementioned weldable alloys, the highest yield strength attained is about 44 ksi in the case of 7005-T6.
Bicycle frames manufactured from higher strength aluminum alloys than the above-mentioned ones have not been utilized for welded construction. Alloy variants 7075 and 7050, which display much higher yield strengths than conventional weldable alloys, have been utilized for bicycle frames that are constructed via mechanical fastening and adhesive bonding. Alloys such as 7075 and 7050 are difficult to fusion weld and exhibit hot tearing, especially in applications where the weld zone is stressed. Thin tube welding, circular welds and other aspects of bicycle frame construction can impart significant stresses on the weld zone thereby eliminating the consideration of high-strength aluminum alloys for this application. It would be a remarkable achievement to derive an aluminum alloy that can be readily welded while attaining yield strengths equivalent to or greater than high strength alloys such as 7075 and 7050.
Bicycle frames have been fabricated from a number of aluminum alloys, as described. Despite the foregoing, there remains a need for lighter, stronger bicycle frames. One way to achieve this is to utilize higher strength aluminum alloys in frame construction, although the use of such materials has been thwarted by their poor weldability. This invention provides a welded-construction bicycle frame of higher strength than previously used. The higher strength of the alloy can be used by designers to produce lighter weight and/or higher strength frames. U.S. Pat. No. 5,507,888 describes the use of an improved 6xxx alloy for bicycle frame tubing. While this patent describes a bicycle frame and an alloy that is improved over the common 6061-T6 construction, it nevertheless displays only a limited improvement in strength. The patent describes strength and elongation values as given in the following table:
U.S. Pat. No. 5,129,666 describes an aluminum alloy bicycle frame in which the tubes that comprise the bicycle frame are reinforced on their ends and threaded. The tubes are also configured to accept adhesive glue for assembly. While this method can allow for the use of high-strength aluminum alloys it is not pertinent for welded construction, as the topic of this invention is.
U.S. Pat. No. 5,597,529 and U.S. Pat. No. 5,620,652 describe various welding filler alloy and base metals in which the alloy modifications are such that weldability is enhanced. A wide array of compositions are proposed as are a number of welded structures including bicycle frames. However, the actual use of the proposed alloys is not reduced to practice and the method of producing any of the alloy formulations into a suitable tubular member is not described. In the present invention, the specific processing sequence is derived to enable the method of fabricating high strength bicycle frames.
The present invention provides for an aluminum alloy bicycle frame manufactured by welded construction utilizing an aluminum alloy with a yield strength in excess of 70 ksi. The frame can thus be made stronger or lighter weight and enables more design options. Previously, aluminum alloys in this strength regime were relegated to bicycle frames that were fabricated via mechanical fastening techniques due to their poor weldability. The bicycle frame of this invention displays both high strength and good weldability. It is preferred that the alloy of this frame, or the tube stock from which the frame components are manufactured from, is a 7xxx-type, or Alxe2x80x94Mgxe2x80x94Zn alloy. It is further preferred that the alloy be extruded and drawn as a tubular member.
According to the present invention, a bicycle frame is constructed by welding, wherein the yield strength of the aluminum alloy tubular members of said frame is above 70 ksi. The alloy is processed via extrusion and drawing as a tubular member. In a preferred embodiment of the invention, the aluminum alloy tubular members are a 7xxx-type alloy.